The active noise reduction functionality relevant to the present invention is realized using “feedback” or hybrid (a combination of feedback and feed-forward) control architectures, in which a or a plurality of sensors which include but are not limited to a microphone is located inboard (i.e. closer to the wearer's ear) of the “receiver” (miniature loudspeaker or driver) in the device. The output of the microphone is used to provide the observation required for feedback (or equivalent) control of the pressure in the ear. Those skilled in the art will understand that systems using pure “feed-forward” controllers do not require the presence of such an inboard microphone.
The in-ear device typically has a housing in which the driver and microphone are located, and which provides an acoustic path from the driver to the outlet of the in-ear device. The outlet is in use located in the ear canal, so that the acoustic signal from the outlet can be delivered to the tympanic membrane (also known as the ear drum).
Positioning of a sensing microphone inboard of the driver requires the microphone is located in the acoustic path between the driver and the outlet. Thus the sound generated by the driver is required to pass around the partial obstacle constituted by the microphone (the body of which is acoustically opaque) in travelling to the ear drum. In existing constructions sufficient space is left around the microphone so that there is no significant acoustic impedance.
The considerations at hand for designing in-ear devices having active noise reduction using feedback control are very different to those present with headphones, or feed-forward architectures. In particular, stability is an issue as the load condition of the device can greatly affect the open loop transfer function (OLTF). This dynamic is the main constraint during the design of the active noise reduction functionality performance of the device. In fact, the larger the dynamic of the OLTF the larger the stability margin of the closed loop system must be in order to ensure the robustness of the active noise reduction functionality.
Further miniaturisation is only exacerbating these issues. For example, commonly used electrodynamic drivers see their source impedance increasing with the inverse of the square of the diaphragm area. Additional measures must therefore be taken to ensure the stability and performance of the system since they become increasingly sensitive to their loads. In addition, moving the electronics required to create a feedback controller, or part thereof, inside the housing would require considerable miniaturisation of the acoustic system to achieve a decrease in overall size of the device and consequently a carefully tuned OLTF would be required to reduce the necessary controller complexity to design a high performance noise cancellation system. The use of acoustic impedances as described in embodiments of the present invention provides a solution for tuning the OLTF for a miniaturised in-ear device incorporating active noise reduction through feedback or hybrid control architectures.